In vitro Activity of Three Fluoroquinolones against Enterococci and <i>Pseudomonas aeruginosa</i>: Modified Categorization System for Discrepant Broth Dilution and Agar Disk Diffusion Test Results

Abstract

The fluoroquinolones ciprofloxacin, norfloxacin, and ofloxacin were examined in vitro against 103 enterococcal isolates and 138 isolates of Pseudomonas aeruginosa, in an attempt to determine which respective proposed interpretive criteria (minimal inhibitory concentration (MIC) versus diameter of agar disk diffusion inhibition zones) correlated best, i.e., resulted in fewer discrepant results. A previously proposed system for grouping discrepant results (very major, major, and minor discrepancies) of Barry and co-workers was modified to comprise 6 categories (very major, major, minor, slight, minimal, and negligible); this expanded system rendered the encountered discrepant in vitro test results more transparent. Overall, the currently employed interpretive criteria for norfloxacin (MICs versus 10-μg disks) resulted in fewer discrepancies with the above two groups of bacterial isolates than those proposed for ciprofloxacin and ofloxacin (MICs versus 5-μg disks, respectively).

Antibiotic Susceptibility Testing (Agar Disk Diffusion and Agar Dilution) of Clinical Isolates of <i>Corynebacterium jeikeium</i>

“…MICs were recorded according to NCCLS criteria [42] where applicable; fosfomycin and mupirocin test data were evaluated in accordance with the criteria of Traub and Spohr [45] and Finlay et al [46], respectively. Discrepancies between agar dilution MIC and agar disk diffusion test results were scored according to the criteria of Barry et al [47] as modified by Traub et al [48].…”

Fluoroquinolone antimicrobial agents.

“…Discrepancies have been noted, however, for some streptococci and Pseudomonas spp. (29,777), among different quinolones (777), and with different lots of Mueller-Hinton agar (251).…”

Antibiotic Susceptibility of Clinical Isolates of <i>Streptococcus pneumoniae</i>

“…However, Jorgensen et al [54] employed MHB with added lysed horse blood for microtiter broth dilution tests, whereas we used Bl-MHA for the agar dilu tion method. In our study, the number of major and minor discrepancies was limited using either the categorization system of Bar ry et al [47] or a modification thereof [48], Therefore we propose that the criteria for tetracycline be considered valid for doxycycline and those for vancomycin be adopted for teicoplanin as well. …”